In the recovery of oil from oil-bearing subsurface formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subsurface formations. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing formation. As the water moves through the formation, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous flooding agent; that is, the injected aqueous agent tends to channel through certain portions of the formation as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors, such as differences in the mobilities of the injected aqueous agent and the displaced reservoir oil and permeability variations within the formation which encourage preferential flow through some portions of the formation at the expense of other portions.
Various techniques have been proposed in order to improve the sweep efficiency of the injected aqueous agent and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected aqueous agent in order to increase the viscosity thereof and thus decrease its mobility to a value equal to or less than the mobility of the displaced oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Many polymeric thickening agents have been proposed for use in such mobility control operations, see U.S. Pat. Nos. 3,085,063; 3,984,333; 3,969,592; 4,110,232; and 4,222,881.
Another procedure of recovering oil from subsurface oil-bearing formations is disclosed in U.S. Pat. No. 3,224,228 and involves alternately flooding the formation with gas and water. Gas is injected into a watered-out formation or the waterflooded portion of an existing waterflooding operation until a desired gas saturation exists in the reservoir. Water is then injected and the gas-water injection cycles repeated until the water-oil ratio becomes uneconomical. Other alternating gas-water injection methods are described in U.S. Pat. Nos. 3,525,395 and 3,529,396. The theory behind these alternating gas-water injection methods is that the gas would cause relative fluid mobility barriers in the permeable streaks such that the following water could flood out more of the oil zone. The problem with this theory had to do with the injected gas channeling so badly due to its low viscosity that the benefits to oil recovery were limited.